1. Field of the Invention
The present invention relates to a packaging method for a flip-chip type semiconductor device, and more particularly to a method of connecting the electrodes of a semiconductor chip to a printed circuit board by soldering.
2. Description of the Related Art
A flip-chip type semiconductor device in which a plurality of solder bump electrodes formed on the surface of semiconductor chips are connected directly to a wiring pattern of a circuit board is broadly in use in the LSI field for the reasons that it permits high density packaging and enables one to obtain high bonding reliability. The technology for this device is described in, for example, an article by Sato et al. entitled "Micro-solder Bonding Technology for IC-LSI", Processings of the Japan institute of Metals, Vol. 23, No. 12 (1984), pp. 1004-1013.
In accordance with the prior art, solder bonding is carried out by applying soldering flux to a circuit board with a wiring pattern formed thereon, positioning the solder bump electrodes of semiconductor chips to the pads of the wiring pattern and binding them with soldering flux, then melting the solder (referred to reflowing hereinafter) by heating. In this process, the soldering flux has an important action of temporarily binding the positioned semiconductor chips by virtue of its bonding power, in addition to the action of smoothly carrying out reflowing.
On the other hand, accompanying the increase in the level of integration of LSIs, heat dissipation from the semiconductor chips of the flip-chip type semiconductor device has become an important technical issue, which necessitates the provision of a heat dissipating means on the rear surface of the semiconductor chips. As one of such heat dissipation methods Rao R. Tummala published an article entitled "Advance Packaging Technologies in U.S.--An Overview", IMC 1988 Proceedings, Tokyo, May 25-27, 1988, pp. 12-17 in which he proposed a structure wherein a cooling piston is pressed mechanically against the rear surface of one semiconductor chip by means of a spring. In addition, R. Darveaux, et al. propose a method in which a heat sink is commonly fitted to the rear surfaces of a plurality of semiconductor chips via a piece of indium with certain thickness in an article entitled "Thermal/Stress Analysis of a Multichip Package Design", Proceedings of the 39th Electronic Components Conference, May 1989, pp. 668-671.
However, the conventional method of temporarily binding semiconductor chips using soldering flux generates problems related to the bonding reliability and the yield due to the fact that many harmful impurities contained in the soldering flux are brought into contact with the front surface of the semiconductor chips. In particular, in view also of the circumstance that the reduction of intrusion of harmful impurities into various materials becomes severe in proportion to the advance of the level of integration and fine geometry of semiconductor chips, it is necessary to exclude soldering flux from the fabrication process.
Moreover, according to the above-mentioned conventional method the rear surfaces of various semiconductor chips, after reflow, do not find themselves located on the same plane, and the height deviation among the rear surfaces of the plurality of semiconductor chips becomes large. This is the reason why the cooling means with long heat dissipation path and complicated structure has to be arranged for each of the semiconductor chips as mentioned above. On the other hand, the aforementioned method of commonly fitting a heat sink by canceling the difference in the height among the plurality of semiconductor chips by forming the indium piece to have a large thickness (500-800 .mu.m, for example) is not desirable because it results in the increase in the thermal resistance up to the location of the heat sink.